avcodec/rv34: Delay failure return for B frames with a missing reference

Failing earlier causes the context to be insufficiently initialized which
can break decoding future frames with threads

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/rv34.c

@@ -1645,6 +1645,7 @@ int ff_rv34_decode_frame(AVCodecContext *avctx,
     int slice_count;
     const uint8_t *slices_hdr = NULL;
     int last = 0;
+    int faulty_b = 0;
 
     /* no supplementary picture */
     if (buf_size == 0) {
@@ -1682,7 +1683,7 @@ int ff_rv34_decode_frame(AVCodecContext *avctx,
         si.type == AV_PICTURE_TYPE_B) {
         av_log(avctx, AV_LOG_ERROR, "Invalid decoder state: B-frame without "
                "reference data.\n");
-        return AVERROR_INVALIDDATA;
+        faulty_b = 1;
     }
     if(   (avctx->skip_frame >= AVDISCARD_NONREF && si.type==AV_PICTURE_TYPE_B)
        || (avctx->skip_frame >= AVDISCARD_NONKEY && si.type!=AV_PICTURE_TYPE_I)
@@ -1772,6 +1773,8 @@ int ff_rv34_decode_frame(AVCodecContext *avctx,
                "multithreading mode (start MB is %d).\n", si.start);
         return AVERROR_INVALIDDATA;
     }
+    if (faulty_b)
+        return AVERROR_INVALIDDATA;
 
     for(i = 0; i < slice_count; i++){
         int offset = get_slice_offset(avctx, slices_hdr, i);